Add contributing guide and pull request template

[eitamring]

Last PR before v0.1.1. Docs only.

• CONTRIBUTING.md: build/test/bench commands and the ground rules the repo already runs on (table tests behind behavior, committed benchmarks behind numbers, one-line commits).
• .github/pull_request_template.md: two comment prompts, nothing to fill out ritually.
• One deliberate line in CONTRIBUTING under development notes discloses AI-assisted tooling, stated once on our terms; strike it if you want it gone.